1. Field of the Application
The present application relates to robotic manipulators and, more particularly, to such manipulators constructed to control the movement of an end-effector with at least two translational and two rotational degrees of freedom, and composed of articulated arms connecting the end-effector to a base.
2. Background Art
Various designs of robotic manipulators exist for controlling the movement of an end-effector with six or fewer degrees of freedom (DOFs). Among these designs, so-called parallel manipulators are becoming increasingly popular.
In a parallel manipulator, an end-effector (i.e., moving portion, output portion) is connected to a base through at least two articulated arms. Often, the joint of each articulated arm that connects directly to the base is actuated. Thus, the actuators of a parallel manipulator are usually fixed to the base, and this reduces significantly the weight of the moving parts of the parallel manipulator and allows higher accelerations at the end-effector.
One of the less-discussed advantages of parallel manipulators is the versatility of their designs. There exists an optimal design for every particular motion pattern. There are parallel wrists for applications that require orienting an object. There are parallel manipulators for applications that require positioning with one orientational (i.e., rotational) and three translational DOFs, and so on. There are also parallel manipulators for applications that require a specific workspace volume (for example, an unlimited rotation about an axis or large displacements along a given axis, etc.). Often, these parallel manipulators comprise five-bar mechanisms.
U.S. Pat. No. 6,047,610, issued Apr. 11, 2000 to Stocco et al., entitled “Hybrid Serial/Parallel Manipulator,” discloses a manipulator having three translational and two rotational DOFs. It consists of two five-bar mechanisms (each actuated by two motors) that pivot about the axes of their base bars. The two five-bar mechanisms are connected to the end-effector through universal joints. It basically registers the position and orientation of a handle, but not the rotation about the handle's axis. This manipulator is redundantly actuated, which inevitably increases the complexity of the control system and, most importantly, the manufacturing cost of the whole unit.
U.S. Pat. No. 6,116,844, issued Sep. 12, 2002 to Hayward, entitled “Mechanisms for Orienting and Placing Articles,” discloses a parallel manipulator having one translational and three rotational DOFs. The parallel manipulator has two five-bar mechanisms (each actuated by two motors) that pivot passively about the axes of their base bars and are connected to a T-shaped end-effector whose extremity runs through a universal-cylindrical joint assembly. In this manipulator, none of the motors is fixed to the base.
U.S. Pat. No. 5,673,595, issued Oct. 7, 1997 to Hui et al., entitled “Four Degree-of-Freedom Manipulator,” discloses a parallel manipulator composed of two five-bar mechanisms moving in parallel planes, whose extremities are connected through a serial triple-revolute-joint chain. A body attached at the middle revolute joint of this chain would therefore have one rotational and three translational DOFs. In this manipulator, an end-effector such as a handle (as in a haptic device) or a suction cup (as in a material-handling robot) cannot be easily attached at the middle revolute joint of the triple-revolute-joint chain. Furthermore, this manipulator has a lock-up singularity when the axes of the extremity joints of both five-bar mechanisms coincide.
U.S. Pat. No. 6,339,969, issued Jan. 22, 2002 to Salcudean et al., entitled “Three-Degree-of-Freedom Parallel Planar Manipulator,” discloses a parallel manipulator having three DOFs in a plane. The manipulator is composed of two five-bar mechanisms moving in parallel planes, whose extremities are connected through a link of constant length. This manipulator registers the position and orientation of a handle moving in a plane. This manipulator is redundantly actuated, which inevitably increases the complexity of the control system and, most importantly, the manufacturing cost of the whole unit.
International Publication No. WO 2006106165, published Oct. 12, 2006, by Nabat et al., entitled “Unlimited-Rotation Parallel Robot with Four Degrees of Freedom,” discloses a parallel manipulator having one orientational and three translational DOFs. The manipulator is composed of two five-bar mechanisms moving in parallel planes, whose extremities are connected through two articulated links, one of which passes through one of the extremities via a cylindrical joint. In this manipulator, the cylindrical joint is subject to lateral forces which increase the friction forces. Indeed, the inventors have eventually constructed a different version of their manipulator, replacing one of the five-bar mechanisms with a more complex mechanical system.
Japanese Publication No. JP2005066723, published Mar. 17, 2005, by Okamoto and Sakaguchi, entitled “Parallel Robot with Four Degrees of Freedom,” discloses a parallel manipulator having two translational and two rotational degrees of freedom. The manipulator is composed of two five-bar mechanisms actuated with linear motors and moving in parallel planes, whose extremities have hollow spherical joints through which a tool holder passes. This manipulator uses linear actuators, which make the manipulator more cumbersome. Furthermore, hollow spherical joints are difficult to manufacture and increase the cost of the manipulator.
The publication “A Two Degrees-of-freedom Planar Haptic Interface with High Kinematic Isotropy,” by Frisoli et al. [Proceedings of 1999 IEEE International Workshop on Robot and Human Interaction, Pisa, Italy] presents a parallel manipulator having two translational and two orientational DOFs. The translational DOFs are active, while the rotational DOFs are passive. The manipulator is composed of a single five-bar mechanism. An unactuated so-called remote-center-of-rotation mechanism is mounted on the extremity of the five-bar mechanism and holds a pen-shaped end-effector. The manipulator is used as a haptic pen that can measure and control the motion on a paper plane while measuring the pressure of the pen exerted by the user along the pen axis. A haptic pen is suitable for applications such as teaching drawing or handwriting, or for rehabilitation. In this particular device, the orientation of the pen is not controllable.
Of all the dozens of parallel manipulator designs, very few are aimed at controlling the movement of an end-effector with two translational and two rotational degrees of freedom, as the invention disclosed in Japanese Publication No. J-P2005066723. Such parallel manipulators can be used as haptic devices, for example, to simulate the motion of a pen along a piece of paper, or they can be used as a subsystem of a five- or six-DOF manipulator. For example, the manipulator disclosed in Japanese Publication No. JP2005066723 can be mounted on a vertical axis and thus serve as a 5-axis machine tool.
The publication “Type Synthesis of 4-DOF Parallel Manipulators,” by Qinchuan Li and Zhen Huang [Proceedings of 2003 IEEE International Conference on Robotics & Automation, Taipei, Taiwan], discloses two such 4-DOF parallel manipulators, one of which consists of four identical arms each composed of five revolute joints connected in series. While this manipulator is theoretically attractive, it is of relatively small practical interest, since its design is cumbersome and its proper functioning relies on strict manufacturing tolerances. Furthermore, the kinematics of such a manipulator are complex.